Type of ceramic wirewound resistor?

N_Cook Inscribed thus:

Woods metal is used to secure the spring ! So about 70C.

Must be higher than that, as a rule of thumb or finger I've come across plenty of in service droppers that are too hot to touch more than fleetingly which means 70 C or above. Unfortunately my variable soldering iron cannot go down that low to try this one in front of me. Looks like a lump of metal with thermometer in it, heated on the stove to check this one

N_Cook Inscribed thus:

I've got some resistors with thermal breaks attached kicking about somewhere. If I recall they were used in some TV sets. If I can lay my hands on them I'll have a look and see what values they are. I do recall being warned not to solder them closed, but to replace them with new ones.

Used to be quite common in TV sets (fusible resistor). Have a look at the top left diagram on page 4 of this which shows mechanical drawing:-

Geo

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if page 41 and in thermally cutout view , sprung open, then yes that type. I'll have another look tomorrow in that pdf but I could not see the melt temperature.

Only approximate this. Some flux over the join of this one in front of me and then using the temp read function of my soldering iron before switching off before reaching set point. Approx 130 deg C no cutout , 160 deg cuts out

Geo Inscribed thus:

Yes ! Those are the ones I remember. Looking at the power curves suggests that they could get extremely hot, however the top graph suggests 70C. The body temperature could get to be far more than that depending upon how long and how good the heat conduction to that joint is.

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The KKE Si derating curve maxes out at 150 deg C , so that must be the fusing temp, agrees with this one

Back in the day when I was a TV engineer, they were very common, and known as 'spring-off resistors'. Often used to be used in the feed to the HOP stage, and would spring open when the HOP valve failed. They were usually re-soldered with what we knew just as 'high melting point solder'. I've no idea what actual temperature the stuff was, just that it was issued to us, a few feet at a time, for re-soldering these devices. The Adcola irons that we used (about 50 watts, as I recall ??) struggled a bit with it, but were ultimately capable of making the joint quite nicely.

Arfa

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Are you saying it was somehow solder that needed a lot of heat rather than a lot of temperature?

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Perhaps N-Cook was thinking of the similar over temperature cutouts used to protect transformers such as used in Philips Tape recorders that used a very low temperature 'solder' to hold the spring connection together. This type of 'fusable link' has been used for well over 100 years and uses very carefully formulated alloy as the sensor to operate at remarkably precise temperatures. The fire sprinkler head is one of the earliest mass produced applications of this technique.

Neil S.

Yes, agreed, but Mr Cook is very well aware of those, and the standard advice of not to try to repair them for precisely the reason that it is a very low melting point alloy that is originally used. However, the spring-off resistors were designed to be resettable by resoldering. The principle is not quite the same as the transformer thermal fuses, as with the case of the spring-offs, the resistor element is designed to get hot, and conduct the heat into the joint, via the thin wide conductor that is closely fixed to the resistor's ceramic body

Arfa

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Perhaps N-Cook was thinking of the similar over temperature cutouts used to protect transformers such as used in Philips Tape recorders that used a very low temperature 'solder' to hold the spring connection together. This type of 'fusable link' has been used for well over 100 years and uses very carefully formulated alloy as the sensor to operate at remarkably precise temperatures. The fire sprinkler head is one of the earliest mass produced applications of this technique.

Neil S.

Someone upthread mentiond a currently available type of them